Cosmogenic ³⁶Cl accumulation in unstable landforms: 1. Effects of the thermal neutron distribution

Abstract

Cosmogenic nuclides produced in situ within minerals at the surface of the Earth are proving to be an effective means of assessing geomorphic histories. The use of multiple cosmogenic nuclides permits both exposure times and erosion rates to be determined. However, if two nuclides are produced only by spallation reactions, the systematic differences in their accumulation rates depend only on the differences in their production rates and half‐lives. The relatively small differences that result require a high degree of analytical precision to yield useful results. In contrast to other spallogenic nuclides, 36Cl is also produced by low‐energy neutron absorption, which creates a different pattern of production as a function of depth. We have measured the thermal flux with depth in a concrete block using 3He‐filled neutron detectors. The measured thermal neutron profile agrees well with predictions from a simple diffusion‐ based thermal neutron distribution model. Calculations of 36Cl production using the model suggest that the use of 36Cl along with a purely spallogenic nuclide to determine erosion rates and exposure times should be less sensitive to analytical error than are determinations from two purely spallogenic nuclides.

The effects of a hydrogen-rich ground cover on cosmogenic thermal neutrons: Implications for exposure dating

“…5). This behaviour is in agreement with model calculations Schimmelpfennig et al, 2009) and measurements (Liu et al, 1994) that predict that the thermal neutron flux in granite increases with depth, reaching a maximum at between ~20-25 cm and declining at depths > 30 cm (Fig. 5).…”

“…2). The dimensions of the granite piles were chosen so that the lateral distance from the neutron counting tubes to the outer surface of the sides of the piles was at least six times the average thermal neutron diffusion length (23 g/cm 2 (Liu et al, 1994); equating to ~9 cm in granite with a measured density of 2.64 g/cm 3 ; see also Fig. 1).…”

“…1). Due to practical and financial considerations we did not build a larger granite pile of the dimensions comparable to the concrete pile used by Liu et al (1994); however, we note that the dimensions of the piles used are not unlike those of commonly sampled features for exposure dating (e.g. moraine boulders).…”

“…It is important to note that our experiments used bare 3 He counters (as did Liu et al 1994), whereas previous experimental studies of environmental neutrons aimed at the effect of water/moisture at the ground/air interface used 3 He or BF 3 counters surrounded by a 2-2.5 cm Polyethylene moderator (Kodama, 1980;Kodama et al, 1985;Zreda et al, 2008). Such moderated counters have an almost constant response to neutrons in the energy range of 1eV-1MeV (Kodama et al, 1985;Yamashita et al, 1966) and suppress incoming thermal neutrons (diffusion length of thermal neutrons in polyethylene is 2.5 cm; Hatton and Carmichael 1966), whereas, bare 3 He neutron detectors are mostly sensitive to thermal neutrons (~0.025 eV) and have a steeply declining response to higher energies (Goldhagen et al, 2003;Vega-Carillo et al, 2005).…”

“…Cosmogenic nuclides with significant production by thermal neutrons are Ca, respectively (Audi et al, 2003;Dunai, 2010;Dunai et al, 2007;Liu et al, 1994;Nishiizumi et al, 2000).…”